1. Technical Field
The present invention relates to the projection of varying magnetic fields, particularly the projection of a substantially linear magnetic gradient field for use in Nuclear Magnetic Resonance Imaging (MRI).
2. Related Art
MRI systems are well-known, and are in commercial use, providing high-quality images in a variety of applications, most notably medical imaging. Typically a sample to be imaged is placed in a strong static magnetic field, for example generated by a superconducting magnet, on which relatively small linearly-varying magnetic fields are imposed. The linearly-varying magnetic field is commonly termed a gradient field, and is used to provide spatial encoding of responsive species within the sample. As is well known, the static magnetic field has field strength of the order of a Tesla, giving rise to a nuclear magnetic resonance frequency of the order of tens or hundreds of megahertz (MHz), the gradient field being much smaller, producing a frequency shift from tens or hundreds of hertz up to a few hundred kilohertz.
Since the basic technology is well-known, it will not be discussed further; reference may be made to Mansfield and Morris “NMR Imaging in Biomedicine”, 1982, Academic Press, the subject matter of which is incorporated herein by reference.
A problem with conventional systems, however, is that it is normally necessary for the sample, or at least a substantial proportion thereof, to be enclosed within a magnetic field-generating system. This can be restrictive, particularly where, for example, imaging is required to assist in surgery, as the surgeon's access to the patient may be restricted during imaging.
Some systems have been proposed to overcome these drawbacks. In one known system, a magnet is moveable along a fixed rail. This enables an image of a patient to be taken and then the imaging apparatus can be moved away for the surgeon to operate on the patient. Drawbacks of this arrangement are that it is bulky and cumbersome to operate, and interactive imaging is not possible while the surgeon is operating; surgery must be suspended while imaging is in progress.
Other proposals have included a magnetic field-generating system in which the field-generating arrangement is split into two portions, each portion located on either side of the region of interest (either horizontally or vertically). This allows more access to the region while imaging is being conducted, but the presence of field-generating equipment either side of the region again presents a restriction on access to the patient.
As a further alternative, a system in which a relatively narrow field-generating system is employed has been proposed. This allows access to the region of interest from either side of the field-generating system, but still encloses the sample and therefore again restricts access to the region of interest to a considerable degree.
It would be desirable to provide a system in which a magnetic field-generating system were located substantially at one side of a patient or sample to be imaged. This would allow much greater freedom of access to the region of interest.
U.S. Pat. No. 5,677,630, the disclosure of which is incorporated herein by reference, discloses a magnetic resonance imaging magnet having a superconductive coil assembly in which a sample volume to be imaged can be located on one side of magnetic field-generating system.
However, if the static magnetic field generator of U.S. Pat. No. 5,677,630 were used with conventional gradient field generators to generate a linear field gradient, the problem of restricted access to the sample would still occur because a gradient field generating assembly surrounding the sample would still be necessary; the document does not disclose any practically achievable means for generating the gradients required for imaging.
EP-A-610012 apparently proposes a projected magnetic field for single-sided nuclear magnetic resonance imaging. In this disclosure, however, no details of the arrangements for generating the necessary static and gradient fields are disclosed, and it is believed that, if an attempt were made to put this disclosure into practice, problems owing to the irregular nature of the field and in particular the non-linear gradients expected from the disclosed arrangements would be encountered.